1. Field of the Invention
The invention relates to a power transmission device.
2. Description of the Related Art
A power transmission device transmits power from one device to another. Some power transmission devices include a plurality of gears engaged with each other. Four-wheel drive vehicles and large six-wheel drive vehicles, for example, have a power transmission device that distributes driving force between the front wheels and rear wheels.
The power transmission device includes three gears: a drive gear to which power from an engine is input, an idler gear for that engages the drive gear, and a driven gear that engages the idler gear.
Japanese Utility Model Application Publication No. 06-025652 describes a lubrication system of a power transmission device in which an oil input groove and an inlet port of an oil drain groove are provided across from each other in a radial direction of an input gear, and in which the inlet port of the oil drain groove opens at a position that is at generally the same height as a part from which oil is introduced from the oil input groove to an oil retaining part of a rear cover.
Gear noise may be occasionally produced when the gears provided in the power transmission device are engaged with each other to be driven. Therefore, techniques for reducing gear noise have been known.
Japanese Patent Application Publication No. 2004-125054 describes a forged gear in which teeth which are obtained by changing at least one of the tooth thickness and the pressure angle of a reference tooth profile within the range of accuracy required for the teeth, are arranged along with the reference tooth profile. The gear obtained by this construction has teeth each having changed profiles and can reduce gear noise.
In addition to the above publication, there have been disclosed methods for reducing gear noise produced when gears are engaged with each other to be driven (see Japanese Patent Application Publication No. 2005-061487, Japanese Patent Application Publication No. 2006-144995, Japanese Patent Application Publication No. 2000-220726, Japanese Patent Application Publication No. 2002-235836, Japanese Patent Application Publication No. 2002-235837, and Japanese Patent Application Publication No. 2001-263453, for example).
In the power transmission device including three gears, the drive gear as a first gear is engaged with the idler gear as an intermediate gear, and the idler gear is engaged with the driven gear as a second gear. Although the methods described in the publications mentioned above reduce gear noise due to engagement between gears, there is still room left for improvement in the structure of the power transmission device, in which gear noise produced between the first gear and the intermediate gear and gear noise produced between the intermediate gear and the second gear occasionally amplify each other to increase the total gear noise.